Flywheel energy storage (FES) provides a renewable way to store energy. Energy from a source such as wind or solar may be stored in a spinning flywheel. This kinetic energy may be used during the periods when the sun isn't shining or the wind isn't blowing. An array of FES units may also supplement the power grid by storing energy at night when demand is low, then augmenting the grid during peak demand.
The kinetic energy stored in an FES is typically harvested using a dynamo, which converts the stored kinetic energy into electricity. The flywheel will slow down as kinetic energy is extracted, and then replenished energy when external energy becomes available. The cycle continues in renewable fashion.
Friction is the greatest impediment to the efficacy of FES. A flywheel spun up to speed will slow down over time as friction slowly bleeds kinetic energy from the system. The bearings that support the flywheel are the main source of friction. To a lesser extent, friction from air resistance will slow the flywheel, but this is easily overcome by placing the flywheel in a vacuum. Because friction within the bearings is a function of force, most FES systems support the axial weight of the flywheel magnetically. A typical configuration uses permanent magnets balanced with electromagnets. Electromagnets require power, however, and are not an optimal solution. Radial forces are typically stabilized with mechanical centering bearings. Ceramic bearings are popular for this purpose as they are more durable, lighter in weight, and do not require lubrication.
The amount of energy stored in a flywheel depends on the mass, velocity, and shape of the flywheel. The simple formula for kinetic energy is E=½mv^2, so doubling the mass doubles the kinetic energy, but doubling the velocity quadruples the kinetic energy. For this reason, commercially available FES devices spin at tens of thousands of rotations per minute. These high angular velocities can be dangerous so flywheels are generally placed within a reinforced container, and the flywheels are made of composite materials that can withstand high force. Shape also plays a role, and more energy is stored when the mass of the flywheel is distributed farthest from the axis of rotation. For this reason, the ideal flywheel is barrel-shaped and open in the center rather than the same mass distributed in the shape of a solid cylinder.
Conventional rotary coil motors are well-known and have been in existence for well over a century, the basic design feature being a rotor ring with ferromagnetic elements passing through a series of stator coils arranged in a circle or toroid. Various methods for transfer of torque have been employed, most commonly using a system of gears, chains, or pulleys. These devices, however, have not enjoyed widespread use.
Subsequent designs and improvements sought to transfer torque by magnetically coupling across a magnetically permeable sealed housing. This advance enabled the movement of fluids without contact between the fluids and vulnerable elements within the motor. Examples include a machine for moving wet cement, another for moving coolant within a nuclear reactor, and a centrifugal pump design.
More recent art replaces the ferromagnetic elements (iron elements which are not magnetic, but which respond to magnetic forces) within the rotor with permanent magnets. Whereas a ferromagnetic element can only be attracted into a coil, a permanent magnet can be simultaneously repelled out of one coil and attracted into an adjacent coil, provided that current through one coil is in the opposite direction relative to the other. U.S. Pat. No. 6,252,317 to Scheffer et al. discloses a commutated electric motor with a plurality of permanent magnets on a rotor that passes through coil stators. This typical permanent magnet/coil motor incorporates a rotor ring comprising a series of magnets arranged in alternating magnetic polarity with spaces or non-magnetic elements between the magnets. The magnetic rotor passes through an interrupted series of coils, the interruptions between the coils being necessary for mechanical transfer of power between the rotor and the powertrain. In this device, torque is transferred by means of teeth on the rotor engaging multiple gear wheels.
While conventional coil motors employ permanent magnet rotors and mechanical means to transfer torque, there are inherent inefficiencies and deficiencies in such coil motor designs and means of transfer torque. The most notable among these is the difficulty in transferring mechanical power from a rotor travelling within a set of coils, typically accomplished by means of gears or pulleys making physical contact with the rotor through spaces between the coils. But allowing these spaces limits the number of coils, and hence, the power density of the motor, and introduces an element of friction. Secondly, these devices harvest only the magnetic field within the coils whereas considerable magnetic field is also available outside the coils to perform meaningful work when configured appropriately.
Generators, which could be described as the converse of electric motors, also suffer from similar inherent inefficiencies and deficiencies. For example, U.S. Pat. Pub. 2012/0235528A1 to Axford teaches a toroidal inductance generator employing magnets within a toroidal copper coil being induced to move by magnetically coupled magnets external to the coil attached to an internal combustion motor. Design limitations, however, preclude this generator from also functioning as a motor.
A clutch is a mechanical device for the purpose of rotory power transmission from one drive shaft to another. The driving member is the shaft attached to an engine while the driven member is the shaft that provides rotary power for work. When fully engaged or locked, the driving member and driven member rotate at the same speed. Slippage occurs when the driving member does not fully engage, resulting in the driven member rotating at a slower speed than the drive member. A brief period of slippage allows for the smooth transition between engagement and disengagement. In the case of the friction clutch, slippage quickly generates unwanted heat, which over time can warp clutch surfaces.
When it becomes desirable that the drive member and driven member rotate at different speeds for any length of time, a transmission is employed. This transmission may be accomplished by various means, including a gear box, a torque converter fluid coupling, or a continuously variable system employing a belt drive with expanding pulleys. A transmission is typically coupled to a clutch which may engage or disengage the transmission from the engine.
Bearings are a major source of friction and heat within a motor. A bearing is a machine element that both reduces friction and constrains motion between moving parts. Many types of bearings exist, but the greatest reduction in friction occurs when a magnetic bearing is employed, which supports a load using magnetic levitation. Magnetic bearings permit relative motion with very low friction and mechanical wear, and thus support the highest speeds of all kinds of bearing.
Typical magnetic bearings employ both permanent magnets, which do not require input of power, and electromagnets which provide external stabilization due to the limitations described by Earnshaw's Theorem. An electronic controller receives input from a position sensor, and energizes the electromagnet so as to maintain a predetermined position between the supporting permanent magnets. Electromagnetic attraction or repulsion and electronic positional feedback ar central to balancing permanent magnet forces in order to achieve friction-free magnetic levitation. An obvious limitation is the requirement of external power for the electromagnets.
Magnetic bearings employing magnetic reluctance do not require external power. Review of the prior art, however, indicates that magnetic reluctance bearings have not enjoyed widespread use. Magnetic reluctance comes into play whenever a group of magnets and ferromagnetic elements are arranged in a circle, allowing completion of a magnetic circuit. Reluctance is said to be at a minimum when a magnetic circuit employs materials with the greatest permeability and when the path of the magnetic flux completes the magnetic circuit by the most direct route possible.
The magnetic permeability of a material is a measure of its ability to allow the passage of magnetic flux. It is analogous to conductivity in electricity. Iron, for instance, has a high magnetic permeability whereas air has low magnetic permeability. Magnetic flux will still pass through air, just as an electric spark will cross an air gap, but flux passes much more readily through iron.
Reducing air gaps between the magnets and/or ferromagnetic components minimizes reluctance. Conversely, reluctance increases whenever a magnetic circuit is disrupted by an increased air gap between the magnetic materials comprising the circuit. Air, having relatively low magnetic permeability, resists the flow of magnetic flux. Directing or focusing the path of flux between the magnetic elements by use of magnet arrays such as the Halbach series facilitates completion of a magnetic circuit and also minimizes reluctance.
Magnetic reluctance has different and advantageous physical and mathematical properties in comparison to the typical magnetic forces of magnetic attraction and repulsion. Whereas the force between magnets falls off with the inverse of the square of the distance between the magnets, reluctance forces increase in a linear fashion with displacement. For example, when two Halbach series are magnetically coupled across an air gap of distance X, the force between the arrays is only ¼ as strong at a gap distance of 2X. Experimentation has shown that when two arrays are made to slide past each other at a constant gap distance X, like railway cars on parallel tracks moving in opposite directions, reluctance forces will increase in linear fashion over a short displacement, achieve a maximum, then fall to zero in linear fashion. By way of reference, both a rubber band and a steel spring demonstrate linear force-displacement characteristics. Pulling on either is initially easy but becomes harder the more the rubber band or spring is stretched up to the point of failure.
Flywheel energy storage (FES) provides a renewable way to store energy. Many such devices have been described. Energy from a source such as wind or solar may be stored in a spinning flywheel. This kinetic energy may be used during the periods when the sun isn't shining or the wind isn't blowing. An array of FES units may also supplement the power grid by storing energy at night when demand is low, then augmenting the grid during peak demand.
The kinetic energy stored in an FES is typically harvested using a dynamo, which converts the stored kinetic energy into electricity. The flywheel will slow down as kinetic energy is extracted, and replenished when external energy becomes available. The cycle continues in renewable fashion.
Friction is the greatest impediment to the efficacy of FES. A flywheel spun up to speed will slow down over time as friction slowly bleeds kinetic energy from the system. The bearings that support the flywheel are the main source of friction. To a lesser extent, friction from air resistance will slow the flywheel, but this is easily overcome by placing the flywheel in a vacuum. Because friction within the bearings is a function of force, most FES systems support the axial weight of the flywheel magnetically. A typical configuration uses permanent magnets balanced with electromagnets, as described above. Electromagnets require power, however, and are not an optimal solution. Radial forces are typically stabilized with mechanical centering bearings. Ceramic bearings are popular for this purpose as they are more durable, lighter in weight, and do not require lubrication.
The amount of energy stored in a flywheel depends on the mass, velocity, and shape of the flywheel. The simple formula for kinetic energy is E=½mv^2, so doubling the mass doubles the kinetic energy, but doubling the velocity quadruples the kinetic energy. For this reason, commercially available FES devices spin at tens of thousands of rotations per minute. These high angular velocities can be dangerous so flywheels are generally placed within a reinforced container, and the flywheels are made of composite materials that can withstand high force. Shape also plays a role, and more energy is stored when the mass of the flywheel is distributed farthest from the axis of rotation. For this reason, the ideal flywheel is barrel-shaped and open in the center rather than the same mass distributed in the shape of a solid cylinder.